Noise Correlations in Dual Frequency VECSEL

Transcription

1 Noise Correlations in Dual Frequency VECSEL S. De, A. El Amili, F. Bretenaker Laboratoire Aimé Cotton, CNRS, Orsay, France V. Pal, R. Ghosh Jawaharlal Nehru University, Delhi, India M. Alouini Institut de Physique de Rennes, Rennes, France G. Baili, G. Pillet Thales Research & Technology France, Palaiseau, France I. Sagnes Laboratoire de Photonique & Nanostructures, Marcoussis, France

2 Introduction ü Need Low noise lasers able to carry an RF local oscillator over long distances ü Applications Coherent telecommunications Atom manipulation and probing High resolution spectroscopy Metrology (atomic clocks) Microwave Photonics

3 Example : remote operation of a radar

4 Example : remote operation of a radar

5 Example : remote operation of a radar One would like to locate the antenna far away from the processing station, but: - RF cable : 1 db/m losses 1 RF channel - Optical fiber: 0.3 db/km losses WDM many channels

6 Basic element: microwave photonics link RF Signal fibre fibre LASER emitter receiver RF Signal Electrical modulation Optical modulation Can we optically generate the carried RF frequency?

7 Several approaches to emit two frequencies Two single mode lasers A single laser providing two frequencies frequency difference Δν inherently stable master Laser 1 slave Laser 2 ν 1 ν 2 when phase locked Δν = ν LO Δν ν LO Ref. electrical signal two longitudinal modes ν 1 ν 2 Laser Δν ν LO Ref. electrical signal Laser two polarizations ν 2 ν 1 Pol. 45 Δν ν LO Ref. electrical signal Δν widely tunable large fluctuations of Δν poor tunability of Δν Δν widely tunable DFL : dual frequency laser

8 Operating principle of a DFL Pump Phase anisotropy Δϕ e ν 1 Polarizer 45 ν 2 Beatnote o Active medium L Etalon Output mirror Opt. Rev. 4, 550 (1997) Δ Δ ν = ν2 ν1 = FSR ϕ π where FSR = c 2L The frequency difference is proportional to the phase anisotropy, thus to the birefringence of the intra-cavity crystal

9 Class-B vs Class-A lasers? Arecchi, Lippi, Puccioni, Tredicce (1984) Class B lasers RIN(dB/Hz) Photons τ photon τ photon ~ < τ inversion laser dynamics ó second order filter ó resonant behavior Frequency Fréquence(Ghz) (GHz) Ibiais = 500mA Ibiais = 400mA Ibiais = 300mA Ibiais = 200mA Ibiais = 100mA Examples: laser diodes, solid-state lasers Population τ inversion Class A lasers τ photon >> τ inversion laser dynamics ó first order filter Example : gas lasers ó free of RO Vertical External Cavity Semiconductor Lasers (VECSELs) G. Baili et al., Opt. Lett. 32, 650 (2007) ; J. Lightwave Tech. 26, 952 (2008).

10 Optically Pumped VECSEL: Increase the photon lifetime by decreasing the cavity losses AR 5 QW InGaAs Bragg Grating SiC Half-VCSEL* Isabelle Sagnes, LPN, France. Heat Dissipation Filter Rc = 50 mm Output Coupler Pump 808 nm Detected Voltage (V) 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 Single frequency operation 7.5 GHz Optical frequency Stable emission up to 50 mw for 1 W pumping Cavity Length 45 mm, 1% Output coupler transmission 150 µm étalon Intracavity losses = 2 % è τ p 15 ns > τ c = few ns è Oscillation-relaxation-free Class-A dynamics

11 Low-noise dual-frequency laser birefringent crystal x o y z e ν 1 ν 2 o active medium étalon mirror

12 x o y z Low-noise dual-frequency laser e birefringent crystal ν 1 ν 2 o laser mode = 140 µm active medium e o 100 µm étalon L cav = 45 mm Pump mode = 260 x 300 µm 2 mirror Puissance Optique détectée sur la PD (dbm) Optical power (dbm) e o ,5 1010,0 1010,5 1011,0 Wavelength (nm) Longueur d'onde (nm)

13 Low-noise dual-frequency laser e Variation of the birefringence by thermooptic effect, electro-optic effect, or by rotating the crystal ν 2 ν 1 active medium o Frequency tunable from 380 MHz to 3.7 GHz étalon What is new here? Puissance électrique relative (db) Electrical Relative RF power power (dbm) (db) mirror Frequency Fréquence (GHz) (GHz)

14 Low-noise dual-frequency laser Class-A dual-frequency VCSEL Dual-frequency Er,Yb:glass laser Puissance Relative RF RF power relative(db) MHz 4 khz NOISE PEDESTAL Frequency Fréquence (GHz) G. Baili et al., Opt. Lett. 34, 3421 (2009) Power (dbm) Frequency (khz) M. Alouini et al., Photon. Tech. Lett. 13, 367 (2001) ORIGIN OF THE NOISE PEDESTAL? Low-noise dual-frequency operation! No excess noise due to relaxation oscillations

15 Dual-frequency VECSEL - Short population inversion lifetime è semiconductor active medium - Increased photon lifetime Class-A Laser è cm-long external cavity d SiC Heat sink Bragg Mirror 6 QW InGaAs AR Coating BC Etalon Pump-diode λ p = 808 nm o e Puissance RF relative(db) Relative RF power (db) G. Baili et al., Opt. Lett. 34, 3421 (2009) 3 MHz Noise pedestal Frequency Fréquence (GHz) ½-VCSEL Where does this noise pedestal come from?

16 Origin of the noise pedestal in the beatnote spectrum? PUMP NOISE Population Inversion Noise Intensity Noises Phase/Intensity Coupling (α factor) Optical Phase Noises RF Beatnote Phase Noise Importance of the correlations between the mode noises!

17 Part 1 Two-frequency VECSEL Relative Intensity Noise laser fluctuations o e δf( f ) RIN( f ) = 2 F0 mean photon number 2 RIN 1 RIN 2 Beat-note AO RF amplitude noise RF phase noise Optical phase noise - Comparison between RF and optical phase noises - Propagation of noises - Correlation between noises Intensity Noise Henry Factor Phase Noise Intensity Noise Correlation!( f ) = "#I 1 ( f )#I 2 * ( f )$ #I 1 ( f ) 2 #I 2 ( f ) 2 Correlation amplitude spectrum: Correlation phase spectrum:

18 Experimental Setup d SiC Heat sink Bragg Mirror 6 QW InGaAs AR Coating BC Etalon Fiber R = 25 mm T = 0.5 % λ/2 IO BS λ/2 RFA PBS D Pump-diode λ p =808 nm D RFA ½-VCSEL D FPI Oscilloscope Oscilloscope Correlation Spectrum

19 Experimental Results 1 mm thick birefringent crystal (BC) d =100 µm d No relaxation oscillation peak Correlation amplitude ~ - 6 db Correlation phase ~ 0

20 Experimental Results 0.5 mm thick birefringent crystal (BC) d = 50 µm d No relaxation oscillation peak Very less correlation (upto 1-2 MHz) Correlation becomes high Arbitrary correlation phase Correlation phase ~ 0

21 Experimental Results 0.2 mm thick birefringent crystal (BC) Change of slope!! d = 20 µm d No relaxation oscillation peak High correlation (~ - 3 db) Correlation phase ~ π Dip!!! Correlation phase ~ 0

22 Part 1 Experimental Results Dual Frequency Laser: Two orthogonally polarized modes oscillating inside same cavity Pumped by same source What we expected? Fully correlated noises (0 db) But what experiment showed!!!!! Partial correlation between intensity noises Dependence of noise correlation on strength of coupling Why? Simple Theoretical Model

23 Rate Equations df dt df dt dn dt dn dt = γ F + k FN = γ F + k F N =Γ( N N ) + k N ( F + ξ F ) =Γ( N N ) + k N ( F + ξ F) Theoretical Model F 1, F : Photon numbers 2 N 1, N : Population inversions 2 1/ γ1,1/ γ : Photon lifetime 2 1/ Γ : Population inversion lifetime k1, k : Proportional to stimulated 2 emission cross-section N, N : unsaturated population inversions 01 ξ, ξ C =ξ 12 ξ 21 Coupling constant Only source of noise: Pump intensity noise : Proportional to cross- to selfsaturation coefficients ratios Identical white noises η <1 Partially correlated ψ = 0 In-phase pump fluctuations Standard linearization around steady-state

24 Comparison C = mm thick crystal C = 0.1 C = 0.1

25 Comparison C = mm thick crystal C = 0.35 C = 0.35

26 Comparison 0.2 mm thick crystal C = 0.65 C = 0.65 C = 0.65

27 Interpretation C = 0.1 C = 0.65 C = 0.35 Low coupling: Dominant in-phase behavior in all frequency High coupling: Dominant anti-phase below cut-off (1-2 MHz) Dominant in-phase above cut-off Moderate coupling: Comparable in-phase and anti-phase behavior below cut-off Dominant in-phase behavior above cut-off Response of Mode 1 or 2 = Anti-phase + or - In-phase

28 Discussion C = mm thick crystal C = 0.1 C = 0.1

29 Discussion 0.2 mm thick crystal C = 0.65 C = 0.65 C = 0.65

30 Discussion C = mm thick crystal C = 0.35 C = 0.35 Syamsundar De et al., Opt. Expr. 21, 2538 (2013)

31 Conclusion Experimental demonstration of intensity noise correlation Dependence of noise correlation on strength of coupling Excellent agreement of simple theoretical model with experiment In progress RF phase noise measurement Removal of noise pedestal Improvement of RF beat note Similar measurements in a class-b solid-state DFL

32 THANK YOU